A spatial light modulator (SLM), as is known, is a device which converts an input light beam, incident on a surface of the device, to an intensity or phase modulated output light beam in response to an electrical or optical input signal. A transmission mode SLM alters the optical absorption and/or index-of-refraction of the device as light passes through it, and uses the remaining light beam as the output beam (as apposed to projecting the input light at an angle to a surface and having the reflected portion of the beam be the output). An SLM, as is known, is very useful for one-dimensional and two-dimensional optical processing, including: matrix multiplication, spatial correlation, and Fourier transformation. It is also known that a charge coupled device (CCD) may be used with a multiple quantum well (MQW) region (described hereinafter) to make an SLM, as described in the article: W. D. Goodhue et. al., "Quantum-well Charge-coupled Device For Charge-coupled Device Addressed Multiple-quantum-well Spatial Light Modulators", Journal of Vacuum Science and Technology, Vol. 4, No. 3, (May/June 1988).
A CCD, as is known, transports an input charge from one temporary storage site to another, at or slightly beneath the surface of a semiconductor. The charge is transferred from one location to the next by electrical clock pulses applied to a series of electrodes mounted to the surface of the CCD.
An MQW, as is known, is a region comprising alternating semiconductor layers, such as gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs, also written as Al.sub.x Ga.sub.1-x As or (Al,Ga)As). The layers of the MQW region have a band-gap energy (i.e., the energy between the valence band and the conduction band for a given layer) pattern which alternates from one layer to the next, also known as multiple quantum wells (MQWs).
To create an SLM with a CCD, the MQW region is placed below the charge transportation area of the CCD. When a given input charge magnitude is above the MQW region, it invokes an electric field within the MQW, which causes the optical absorption coefficient of the MQW to change in response thereto. Thus, if a light beam is passed through the MQW, the magnitude of the charge present at a given time will determine the absorption of light by the MQW. More specifically, it is known that an electric field applied within the MQW changes the quantum energy levels of electrons and holes, which affects its absorption characteristics.
It is also known that the optical absorption coefficient (.alpha.) of a semiconductor varies as a function of the optical energy (Eo) of the photons of the incident light. The optical energy Eo is defined as: Eo=hv=hc/.nu., where h is Plancks constant; .lambda. is the frequency, c is the velocity of light; and .lambda. is the wavelength of the incident light. Thus, the absorption coefficient .alpha. varies as the optical wavelength .lambda. varies. If the energy Eo of the photons is below the band-gap energy (Eg), also known as the absorption band edge, for the semiconductor, minimal photons are absorbed thereby. Conversely, if the energy Eo of the photons is greater than the band-gap energy Eg, the semiconductor will readily absorb the photons.
The MQW region, as is known, provides an optical absorption characteristic which can be treated as having a effective collective energy band-gap (more precisely called an optical absorption threshold) for the entire MQW region, between the bulk band-gaps of the two semiconductors used, i.e., larger than GaAs and smaller than AlGaAs, which varies with applied electric field strength. The MQW absorption threshold is due to the quantum-size and two-dimensional excitonic effects, as is known. Its variation is known as the quantum confined Stark effect which is due to a shift in excitonic absorption with applied electric field, as described in the article: Miller et al, "Electric Field Dependance of Optical Absorption Near Bandgap of Quantum Wells Structure", Phys. Rev. B, Vol 32, Pg 1043 (1985). Because the MQW absorption threshold is less than the band-gap energy of AlGaAs (the material in the MQW having the larger band-gap) it allows photon with energy smaller than the bulk AlGaAs band-gap to be absorbed by the MQW. Thus, optical energy of the incident light can be absorbed by the MQW but not absorbed by AlGaAs layers external to the MQW region.
Using a CCD to create an SLM has numerous drawbacks. First, the useful optical area of a CCD SLM is severely limited because the CCD requires electrodes to be located on the same surface that the input light is incident on. Also, a CCD requires clock circuits to move the charge from one location to the next along the device, thereby requiring extra circuitry on the substrate or external thereto. Furthermore, a wire or thin film metal interconnect must be connected to each electrode to provide a clocking voltage thereto, thereby requiring many wires and/or interconnects for a high density optical application. Because of these limitations, desirable high density one and two-dimensional SLM's may not be obtained without a severe size penalty and added complexity which decreases yield and increases cost.